There are many methods of rendering grey images on an output device. Moreover, error diffusion can render complex images that contain a mixture of text and picture reasonably well. This utilization of error diffusion eliminates the need to have image segmentation to separate the text from the picture so that the picture aspect of the document can be screened and the text aspect of the document can be threshold.
FIG. 1 illustrates a typical error diffusion technique. In Step S1 of this process, the video signal for pixel X is modified to include the accumulated error diffused to this pixel from previous threshold processes. The modified video signal value (X) is compared at Step S2 with the value 128, assuming a video range between 0 and 255. If Step S2 determines that the modified video signal value (X) is greater than or equal to 128, the process proceeds to Step S4 wherein a value is output to indicate the turning ON of pixel X. The process then proceeds to calculate the error associated with the threshold process at Step S6 wherein this error, Y, is calculate as being X-255.
On the other hand, if Step S2 determines that the modified video signal value (X) is less than 128, a signal is output at Step S3 indicating that the pixel X is to be turned OFF. The process then proceeds to Step S5 wherein the error, Y, is calculated as being equal to the value X.
The error calculated in either Steps S5 or S6 is multiplied by weighting coefficients and distributed to downstream pixels in Step S7. Thus, the error from the threshold process is diffused to adjacent pixels. The coefficients conventionally used to diffuse the error to adjacent downstream pixels are illustrated in FIG. 2.
In FIG. 2, X represents the current pixel being thresholded. The weighted error from this threshold process is diffused to adjacent downstream pixels according to preselected coefficients. For example, the weighting coefficient for the next pixel in the same scanline conventionally is 7/16, whereas the coefficient for the pixel that is one over in the fast scan direction and one down in the slow scan direction from the currently processed pixel is 1/16.
This method provides good results, but with advances in marking or printing technology, a new error diffusion method is needed. More specifically, it has become possible to pulse width modulate a laser to print images with high addressability. To use error diffusion in combination with high addressability, one cannot simply perform the error diffusion at the high spatial resolution corresponding to the high addressability because the resulting subpixels would be too small for a typical print engine to render. Thus, it is desired to develop an error diffusion technique which can be effectively utilized with the present day highly addressable image output terminals without producing subpixels too small for rendering.